With the addition of four states during the war and the immediate crisis leading to it, the United States flag went through many changes during the war. Below are the four official flags of the United States used during the war. Note that at the time of the Civil War, there was no standard arrangement for the stars in the union of the flag, thus many different arrangements were used.
With the addition of Kansas to the Union on January 29, 1861, the 34 star flag became the official flag of the United States. However, the garrison at Fort Sumter was still flying this 33 star flag at the time of the bombardment in April 1861. Note the unusual arrangement of the stars.
The 34 star flag was in use through much of the first half of the war.
After the "secession" of West Virginia, and its acceptance into statehood on June 19, 1863, the 35 star flag became the official flag of the United States. This was the most commonly used U.S. flag during the war.
After Nevada joined the Union on October 31, 1864, the 36 star flag became the official flag.
Probably the first flag of this design was the I Corps Headquarters flag used by General John Reynolds, who was killed in action at Gettysburg. His design was used as the basis for many of the other corps flags.
Adopted in 1864, this flag is a modified I Corps flag with the II Corps symbol, the trefoil or shamrock, in the center.
The Army of the Potomac's III Corps used this flag which differed from the I Corps only in the number in the symbol.
The hard-fighting V Corps was very proud of the Maltese Cross, its corps symbol, so it wouldn't do to leave off the corps flag.
After 1864, the VI Corps symbol was the Cross of Saint Andrew. Prior to this it had been the saltier cross.
The Army of the James' X Corps fought in Virginia and North Carolina, under this simple, but effective corps headquarters flag.
The often ill-fated XI Corps of the Army of the Potomac used the I Corps flag design with the number 11 in the center.
The XVIII Corps of the Army of the James fought under this flag, similar to the X Corps.
One of the new corps of the Army of the Potomac, the XIX Corps used this flag
The XX Corps flew this flag during the last year of the war, including their capture of Atlanta, and the March to the Sea.
Another of the new corps of the Army of the Potomac, the XXIV Corps used this flag which featured a heart, their corps symbol.
The XXV flew this flag which featured their corps symbol, the square.
Sometime after taking command of the Union Army, General Grant adopted this flag to mark his headquarters, which was in the field with the Army of the Potomac. A 36 star flag, it has a slightly unusual arrangement for the stars.
The Union hero of Gettysburg, Joshua Chamberlain enjoyed a meteoric rise through the command structure after his heroic actions on Little Round Top. After taking command of a brigade in V Corps, he adopted this banner to mark his location on the battle field.
One of the more flamboyant generals in Union service, the hot-tempered Phil Sheridan used this banner after taking command of the Cavalry Corps of the Army of the Potomac.
Edging Sheridan inthe flamboyant department, 23-year old General George Custer designed this fancy flag for his personal banner. He was still using it in 1876 at the Little Bighorn when he uttered his famous last words, "Hey, where did all these Indians come from?" (artistic license used liberally)
Another of the Union's heroes at Gettysburg, General John Buford was flying a standard U.S. Cavalry guidon as his personal banner when he made his heroic stand north of Gettysburg which bought the Union Army enough time to seize the vital high ground south of town.